Final answer:
Dehydration causes a reduction in blood plasma volume, leading to impaired thermoregulation and cardiovascular function, which detrimentally affects training performance.
Step-by-step explanation:
Dehydration leads to a reduction in blood plasma volume, which can significantly impact training performance. Among the given options, the correct answer is Impaired thermoregulation and cardiovascular function. When the body loses more water than it takes in, it experiences a negative water balance, leading to dehydration. This state results in decreased blood volume, which hinders the ability to regulate body temperature and deliver oxygen and nutrients to the muscles during exercise. Consequently, it may cause an increase in body temperature, exhaustion, and diminished endurance and strength, negatively affecting overall training performance.